The present invention relates to a method of manufacturing a compound material, and more particularly to a method of manufacturing such a material which has a high corrosion resistance.
The corrosion resistance of ferritic chromium steel materials is, in addition to the chromium content and the addition of, for instance, molybdenum, determined by the carbon and nitrogen content in the steel material. While the conventionally manufactured ferritic chromium steel AISI 430 corresponding to SAE 51430 has a lower resistance to oxidizing and reducing acids, bases, and sulfur dioxide and chlorine containing atmosphere than an austenitic chromium-nickel steel AISI 304 corresponding to SAE 30304, its corrosion resistance at the same chromium content improves with the reduction of the carbon and nitrogen content. When the carbon content is between 0.001 and 0.003% and the nitrogen content is not exceeding 0.01%, this chromium steel (superferrite) has corrosion resistance values which are higher than those of the austenitic chromium-nickel steel. In particular, the resistance of this superferritic steel to stress corrosion cracking, intergranular, pitting and crevice corrosion is better than the resistance of the austenitic alloyed steel. Also, the corrosion-resistance in oxidizing and reducing acids, as well as in the atmosphere, is higher than that of the austenitic chromium-nickel steel.
Ferritic chromium steel materials with reduced carbon contents of 0.001 to 0.003% could, according to the recent state of the melting and refining techniques, be manufactured only in electron beam vacuum ovens. The other improved manufacturing methods, such as the vacuum oxygen blow-refining method, the vacuum inductive melting method (VIM) and the argon-oxygen decarburization method (AOD) produce steel materials of carbon contents of between 0.01 and 0.02%. In order to give the "extra low carbon" steel materials having 0.01 to 0.02% of carbon content, a corrosion resistance which equals or exceeds the corrosion resistance of the austenitic steel materials, the carbon and nitrogen contents of the steel material must be chemically reacted, that is, stabilized. In most instances, titanium is used for the stabilization, but other alloying elements, such as niobium and tantalum can also be used for the same purpose.
Experience and tests with ferritic chromium steel materials which are stabilized with titanium have shown that such materials have a substantially higher corrosion resistance than non-stabilized chromium steel materials having a normal carbon and nitrogen content (such as SAE 51430 steel). The titanium content must be at least six times higher than the sum of the carbon and nitrogen contents. The ferritic chromium steel materials which are stabilized with titanium, however, are disadvantageous in comparison with the non-stabilized materials in that they have a lower purity and have a surface of worse quality. The surface cannot be ground to give it a mirror-like appearance.
It has also been recently proposed, in German Pat. No. 26 21 329, to compound a carrier steel material of deep-drawing grade, which is alloyed with a carbide and nitride forming substance, with a ferritic chromium steel material of normal carbon content, to roll the compound steel material to a strip of sheet, and then to anneal the compound steel material of the fine sheet at a temperature and for a time period sufficient for the carbon content of the ferritic chromium steel coating layer to be reduced to between 0.001 and 0.003%; thereby increasing the corrosion resistance of the ferritic chromium sheet material to that of a superferritic material. However, experience has shown that, under many circumstances, it is difficult to perform the annealing operation.